Investigators have disclosed that the flight recorders of the Airbus A320 that crashed off the French coast on 27 November resisted the impact, but that no data from them could be extracted.
There is also no evidence of any unusual circumstances arising from the maintenance procedures undertaken before the flight.
France's BEA accident investigation agency says that the flight recorders' electronic memory cards appear intact, but no information could be retrieved. It says "further work" will be needed on the recorders, but adds that it is unable to predict the results of these efforts.
Maintenance of the aircraft began on 3 November, adds the bureau, and comprised visual and functional inspections as well as repainting of the aircraft, which was being returned off-lease by XL Airways to Air New Zealand.
"An initial review of the documents has not revealed any abnormalities," says the BEA.
Having undertaken flight circuits, the A320 had been returning to Perpignan for refuelling ahead of its delivery departure to Frankfurt. "The crew had given no indication of any problem to air traffic control when [the aircraft] stopped responding to calls," the bureau says. "At this stage of the investigation, nothing can explain why the aircraft went off course and crashed into the sea."
All seven occupants of the A320 are presumed to have been killed in the accident, following the recovery of three bodies. Now that the recorders have been retrieved, the BEA says it will shortly begin pulling up wreckage of the aircraft from its position, 40m (130ft) below the surface of the Mediterranean.
Meanwhile, the A320 accident prompted ANZ to postpone a test flight using biofuel until next year. The flight had been due to take place on 3 December. "Full efforts and resources of Air New Zealand will be focused over the coming days on providing support to the family members of the missing New Zealanders and to our people, and assisting in the investigation of the A320 accident in France," says the carrier.
The biofuel flight will involve a Boeing 747-400 with one of its Rolls-Royce RB211 engines powered by a 50:50 mix of Jet-A and synthetic kerosene derived from the jatropha plant.